sub verify_args {
    my $defaults = shift; # leave the rest of @_ intact.

    # Verify that args came in pairs.
    croak "Expecting key => value pairs, but got odd number of args"
        if @_ % 2;

    # Verify keys, ignore values.
    while (@_) {
        my ($var, undef) = (shift, shift);
        croak "Invalid parameter: '$var'"
            unless exists $defaults->{$var};
    }
}